1. Field of the Invention
The present invention relates to a charging device, and more particularly, to a charging device capable of performing main charging on a secondary battery by supplying a normal current to the secondary battery as a charging current and of performing trickle charging on the secondary battery by supplying a weak current to the secondary battery as the charging current.
2. Description of the Related Art
Conventionally, there has been disclosed a charging device having a function for mainly charging a secondary battery by supplying a normal current (a large current) to the secondary battery as a charging current and a function for trickle-charging the secondary battery by supplying a weak current to the secondary battery as the charging current.
In general, when it is impossible for a secondary battery to generate the original electromotive force due to the deterioration of the performance of the secondary battery, the trickle charging is performed to restore the original performance of the secondary battery.
FIG. 2 is a circuit diagram illustrating a circuit structure of a conventional charging device 1 having the trickle charging function. The charging device 1 has a main charging circuit 3 for performing the main charging on a secondary battery 2 and a trickle charging circuit 4 for performing the trickle charging on the secondary battery 2.
In the main charging circuit 3, a charging current detecting resistor 7 is provided on the downstream side of a terminal 6 of a power source 5, and the output side of the charging current detecting resistor 7 is connected to an inverting input terminal of an operational amplifier 8 through a resistor r1. The operational amplifier 8 controls the value of the charging current while detecting the current flowing through the charging current detecting resistor 7.
Meanwhile, a non-inverting input terminal of the operational amplifier 8 is connected to the ground through a resistor r2. In addition, a resistor r3 is provided between the non-inverting input terminal of the operational amplifier 8 and the input side of the charging current detecting resistor 7.
Further, the output side of the charging current detecting resistor 7 is connected to an emitter of a power transistor Tr1 serving as a power element for controlling the supply of the normal current to the secondary battery 2, and a collector of the power transistor Tr1 is connected to an anode of the secondary battery 2. In addition, a resistor r4 is provided between a base and the emitter of the power transistor Tr1.
Furthermore, the base of the power transistor Tr1 is connected to an emitter of a transistor Tr2 for controlling a base current of the power transistor Tr1, and a collector of the transistor Tr2 is connected to an output terminal of the operational amplifier 8 through a resistor r5. In addition, a resistor r6 is provided between a base and the emitter of the transistor Tr2.
Moreover, an end of a resistor r7 is connected to the base of the transistor Tr2, and the other end of the resistor r7 is connected to a collector of a transistor Tr3 for controlling the base current of the transistor Tr2.
An emitter of the transistor Tr3 is connected to the ground, and a base thereof is connected through a resistor r8 to a power supply terminal 10 for inputting a base current. In addition, a resistor r9 is provided between the base and the emitter of the transistor Tr3.
Further, in the trickle charging circuit 4, a resistor r10 is provided on the downstream side of the terminal 6 of the power source 5, and the output side of the resistor r10 is connected to an emitter of a power transistor Tr4 serving as a power element for controlling the supply of the weak current to the secondary battery 2.
A collector of the power transistor Tr4 is connected to the anode of the secondary battery 2, and a diode 11 is provided between a base of the power transistor Tr4 and the downstream side of the terminal 6 of the power source 5. In addition, an end of a resistor r11, is connected to the base of the power transistor Tr4, and the other end of the resistor r11 is connected to a collector of a transistor Tr5 for controlling the base current of the power transistor Tr4.
A base of the transistor Tr5 is connected to a base current input terminal 12 through a resistor r12. In addition, a resistor r13 is connected between an emitter of the transistor Tr5 and an end of the resistor r12 connected to the base thereof. Further, the collector of the transistor Tr5 is connected to the ground.
According to the charging device having the above-mentioned structure, when the main charging is performed on the secondary battery 2, a base current is supplied to the transistor Tr3 such that the transistors Tr3 and Tr2 and the power transistor Tr1 are turned on.
Then, a normal current of about 700 mA is supplied as the charging current from the power source 5 to the secondary battery 2 through the emitter and the collector of the power transistor Tr1, thereby performing the main charging on the secondary battery 2.
In this case, since a base current is not supplied to the base of the transistor Tr5, the transistor Tr5 and the power transistor Tr4 are maintained in an off state.
Subsequently, when the trickle charging is performed on the secondary battery 2, the base current is supplied to the base of the transistor Tr5 in a state in which the power transistor Tr1 and the transistors Tr2 and Tr3 are off, so that the transistor Tr5 and the power transistor Tr4 are turned on.
Then, a weak current of about 50 mA is supplied as the charging current from the power source 5 to the secondary battery 2 through the emitter and the collector of the power transistor Tr4, thereby performing the trickle charging on the secondary battery 2 (for example, see Japanese Unexamined Patent Application Publication No. 5-284664).
However, in the conventional charging device 1, the main charging circuit 3 is separately arranged from the trickle charging circuit 4. Therefore, the circuits must respectively have expensive power transistors Tr1 and Tr4 for controlling the charging current (the normal current or the weak current), which causes an increase in the costs of manufacturing the charging device.
Further, since the power transistors Tr1 and Tr4 in the respective circuits through which the charging current flows have high heating values, it is necessary that radiators (not shown) for radiating heat generated from the power transistors Tr1 and Tr4 be provided to the power transistors. Tr1 and Tr4 separately, which causes an increase in the number of components and an increase in manufacturing costs.